The present invention relates to a semiconductor device having wiring electrodes and, more particularly, to a semiconductor device wired through contact holes.
In semiconductor devices such as MOS transistors, aluminum is used as a wiring layer. A MOS transistor has, for example, n-type diffusion regions in the surface area of a p-type silicon substrate. These regions are used as a source and drain. Contact holes are formed by patterning an insulating layer covering the substrate and diffusion layers. Wiring layers are formed, extending through the contact holes and contacting the diffusion layers. In connecting the wiring layer with the diffusion layer, Al is deposited on the diffusion layer by sputtering, and alloyed with Si by annealing.
However, when the wiring layer is heated to a temperature of 450.degree. C. in another step, the Al diffuses from the wiring layer into the silicon substrate, growing Al-Si spikes within the substrate. In recent years, shallow diffusion layers are often formed with the use of BF.sub.2 and BCl.sub.2, and, in this case, the alloy spikes may grow deeper than the diffusion layer. If spikes penetrate the diffusion layer and reach the substrate, the pn junction between the substrate and the diffusion layer will break down.
Furthermore, disconnection of the wiring layer occurs easily in the portion inside of the contact hole. This is because electromigration occurs in this portion when a relatively large current flows in the Al wiring layer.
Two conventional methods of preventing alloy spikes are known. In the first method, Al is deposited by sputtering, together with about 1 to 5% of Si (based on Al amount), onto the diffusion layer. This method can reliably prevent alloy spikes. However, it can only be used when an increase in contact resistance can be ignored. In practice, such an increase in contact resistance cannot be ignored, and the Si amount must be reduced to a low percentage at which alloy spikes can not be sufficiently prevented. Namely, a high amount of silicon is preferable in the control of alloy spike growth. If the Si amount exceeds the solid solubility of Al, however, solid Si precipitates in the wiring layer in addition to the Al-Si alloy. The precipitated Si has a remarkably low conductivity. The contact resistance of the wiring layer undesirably increases due to the low conductivity. In particular, the narrow region of the wiring layer inside of the contact hole may be electrically separated from the diffusion region due to the precipitated Si. In addition, Al and Si may not always be deposited at a uniform ratio during sputtering. As a result, even if the Si amount is within the solid solubility of Al, Si will precipitate in a portion of the wiring layer.
In the second conventional method, a barrier metal layer, such as a TiN layer, is formed between the diffusion layer and the Al layer. When the barrier metal is TiN, the boron (B) contained in the diffusion layer diffuses into the TiN layer, making it impossible to obtain good ohmic contact.